Electroluminescence displays are driven by current and/or voltage circuits. An example of a voltage driven display is a liquid crystal display. An example of a current driven display is an organic light emitting display (OLED). Current driven display devices, like most displays, are configured in matrices of pixels that cover a display area. The matrix has rows and columns of pixels, where each pixel in the matrix may be turned on or off to produce patterns of light that constitute the display. Each pixel may constitute one or more diodes that each emits light having a distinct color. With three different diodes having distinct colors, most colors can be reproduced.
There are several problems associated with driving current driven displays which affects the quality of the image that the display produces. One of the problems is how to drive the matrix fast enough to turn pixels on and overcome capacitance of each pixel. Another problem is how to drive the matrix in way that produces pixels having brightness that are uncorrelated with the number of pixels that are “ON” in a given row of the matrix. A phenomenon called cross-talk relates to the effect that ON pixels within a row have on other pixels in the row. Unless corrected, there is a tendency for pixels in a given row to dim as the number of ON pixels increases.
One solution to driving the matrix of pixels fast enough is to use a voltage source in addition to a current source to pre-charge each pixel. The voltage source charges the pixel capacitance of each “ON” pixel. Then, the current source drives each pixel diode after the pre-charge cycle is complete. This solution has the advantage of shortening the time it takes to overcome the capacitance of each “ON” pixel and causes most of the current from the current source to drive the “ON” pixel diodes.
A problem remains, however, because the current from each “ON” pixel empties into a common ground. The common ground has a characteristic resistance associated with it that produces a parasitic voltage as a result of the current from the “ON” pixels. The parasitic voltage is subtracted from the pre-charge voltage and reduces the efficacy of the pre-charge voltage. Moreover, the parasitic voltage increases with each additional pixel that is turned “ON” in a given row. Thus, the quality of the display suffers and pixels appear dimmer as the number of “ON” pixels in a row increases.
Accordingly, there is a need for new technique for pre-charging current driven electroluminescent display pixels that produces ON pixel intensities that are relatively independent of the number of ON pixels in a given row. There is a further need for a technique for combating parasitic voltage induced on common ground lines within matrices of pixels. There is still a further need for display driver that compensates for parasitic voltage and that may be used to drive a range of display devices, each having its own current and parasitic voltage peculiarities.